All operations remained suspended yesterday at the sole facility in the US for storing enriched uranium after the area was breached by three anti-nucl ear protesters, including an 82-year-old nun, exposing gaps in security provided by G4S, the same private company accused of bungling security arrangements for the Olympics.

After cutting through three fences around Y-12, a Second World War-era nuclear weapons complex in Oak Ridge, Tennessee, the three activists, identified as Megan Rice, 82, Michael Wallis, 63 and Greg Boertje-Obed, 57, got as far as the outer wall of the uranium building and allegedly daubed it with slogans and splashed it with human blood.

A spokeswoman for WSI Oak Ridge, which is contracted by the Energy Department to keep intruders out of the highly sensitive complex, declined to respond to questions yesterday. The company is a subsidiary of the international security firm G4S which acknowledged shortly before the London Games that it had been unable to assemble sufficient numbers of staff to keep them safe, forcing the Government to deploy Army troops.

While the incursion has served once again to embarrass G4S, a spokesman for the Oak Ridge Environmental Peace Alliance said that was not the original purpose of the successful protest. "It wasn't so they could show how easy it was to bust into this bomb plant, it was because the production of nuclear weapons violates everything that is moral and good," Ralph Hutchinson told Reuters. "It is a war crime."

The three perpetrators, who seemingly wandered within the perimeter fences of Y-12 for two hours before reaching the key storage building, have been charged with "vandalism and criminal trespass". They were due to appear before a judge in Tennessee later last night for a bail hearing. They are expected to face trial in early October.

All questions to WSI were being referred to Steve Wyatt, spokesman of the National Nuclear Security Administration (NNSA), which is part of the Energy Department. "We're taking this very, very seriously," he said, confirming that the trio had cut through two chain link fences on the edge of Y-12 and a third fence closer to the structure where they left the slogans known as the "Highly Enriched Uranium Materials Facility".

Hidden away in countryside to the west of Knoxville, the Oak Ridge complex was established in 1942 and became ground zero for the "Manhattan Project", which led to the development by the US of the atomic bomb. It remains a mythical place for the peace protest movement.

Officials insist the safety of the uranium at Y-12 was never compromised by the incursion but the fact that the three were able to get so close is nonetheless a cause of deep concern. "It is unbelievable this could happen," noted Peter Stockton, a former congressional investigator and security consultant to the federal government. "The significance is outrageous. If they were terrorists, they could have blown open the door and got inside." He said the security breach was the "worst we've ever seen" in connection with a US nuclear facility.